Starting from the casual observation that neighbouring lines of sight to stars at similar distances in the LISM show abruptly varying column densities, we developed a technique for measuring properties of single clouds. Fig.l shows an idealized version of this. The steps (Trapero et al. 1992, 1995) are:

1. – Correct for extinction the distances to local stars used, via B-V (where available), or our own measured Nal or KI absorptions.

2. – Measure diametral column density towards A from Nal or KI equivalent width and convert to hydrogen: N(H) [cm−2].

3. – Measure cloud diameter, between E and E’.

4. – Estimate the number density, n(H) = N(H)/d [cm−3].

5. – Use a canonical thermal pressure, nT = 3500 K cmThis at least gives the decision cool (<100K) or warm (>5000K).

6. – Estimate the cloud mass by integrating n(H) over the estimated volume.

Following up, cool cores within warm clouds, or warm envelopes around cool clouds can be identified using distance and velocity as tags. We have found empirical rules of thumb to guide us:

1. – The KI resonance at 7699Å is ideal for finding cool clouds; it is too weak to show up in warm clouds.

2. – NaI D doublet lines weaker than 5mÅ typify warm clouds. There are too many of these, by a factor 50, to come from cool cloud edges. This is confirmed by the “low velocity Routly-Spitzer effect” (see below).